1. Field of the Invention
The present invention is directed in general to field of information processing. In one aspect, the present invention relates to a system and method for codeword retransmission within MIMO communication systems.
2. Description of the Related Art
Wireless communication systems transmit and receive signals within a designated electromagnetic frequency spectrum, but the capacity of the electromagnetic frequency spectrum is limited. As the demand for wireless communication systems continues to expand, there are increasing challenges to improve spectrum usage efficiency. To improve the communication capacity of the systems while reducing the sensitivity of the systems to noise and interference and limiting the power of the transmissions, a number of wireless communication techniques have been proposed, such as Multiple Input Multiple Output (MIMO), which is a transmission method involving multiple transmit antennas and multiple receive antennas. For example, space division multiple access (SDMA) systems can be implemented as closed-loop systems to improve spectrum usage efficiency. SDMA has recently emerged as a popular technique for the next generation communication systems. SDMA based methods have been adopted in several current emerging standards such as IEEE 802.16 and the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) platform.
Wireless communication systems, such as the example MIMO system 100 shown in FIG. 1, include one or more transmitters 101 and one or more receiver stations 105.1-105.m, where “m” is an integer representing the number of receiver stations in a given geographic area. As depicted, the transmitter 101 (e.g., base station) uses a first multiple antenna array 104 to communicate with one or more receiver stations 105.1-105.m (e.g., subscriber stations), each having its own receiver antenna array 106.i (e.g., 106.1, 106.2, . . . 106.m), where each antenna array 106.i includes one or antennas. While the wireless communication system 100 may be any type of wireless communication system (including but not limited to a MIMO system, SDMA system, CDMA system, OFDMA system, OFDM system, etc.), an example MIMO wireless communication system 100 includes a transmitter 101 (which may act as a node B or base station) and one or more receivers 105.i (each of which may act as a subscriber station or user equipment), which can be virtually any type of wireless one-way or two-way communication device such as a cellular telephone, wireless equipped computer system, and wireless personal digital assistant. Of course, the receiver/subscriber station 105.i can also transmit signals which are received by the transmitter/base station 101. The signals communicated between transmitter 101 and receiver 105.i can include voice, data, electronic mail, video, and other data, voice, and video signals.
With MIMO communication systems, the transmitter 101 can use knowledge about the communication channel at each receiver 105.i to operate on the transmit signal before transmitting from the transmit antenna array 104 by using precoding techniques, thereby improving the quality and capacity of signal transmission to the subscriber stations 105.i. Precoding is implemented by applying a set of transmit beam forming or precoding weights to signals applied to each antenna in the antenna array 104 prior to transmission. For example, precoding techniques may be used to implement spatial multiplexing whereby independent and separately encoded data signals, so called streams, are transmitted from each of the multiple transmit antennas to a single subscriber station, effectively re-using or multiplexing the space dimension. With spatial multiplexing, the number of layers (or rank) simultaneously transmitted to one subscriber station (e.g., 105.1) may be adapted to match current channel characteristics. In this way, rank adaptation may exploit the potential capacity boost offered by multiple antennas when the transmission channel is rich with multipath diversity.
Examples of such precoding techniques may be understood with reference to FIG. 1, where the MIMO system base station 101 receives one or more information signals (e.g., s1-sm). Each information signal si is encoded, modulated and/or mapped into transmission layers for downlink transmission by the channel encoding/modulation/mapping module 102.i, and then precoded with a precoding vector prior to transmission over the transmit antenna array 104. For example, when precoding is used to implement spatial multiplexing through multiple antennas, one or more transport blocks or codewords may be simultaneously transmitted over one or more layers to the same subscriber station (e.g., 105.1). In this context, a layer is a symbol stream originating from the modulated bits of a codeword, where a codeword refers to the coded bits of one transport block. To spatially multiplex one or more symbol streams (e.g., signal s1 and s2) through multiple antennas, “precoding” weights (e.g., precoding vectors w1 and w2) are applied to one or more symbol streams (e.g., signal s1 and s2) and the result (e.g., vectors x1 and x2) is transmitted over an array of antennas 104. (Note: lower case bold variables indicate vectors and upper case BOLD variables indicate matrices). The precoding vectors wi may be stored in a codebook (not shown) and used to direct the signal with the objective of enhancing the signal quality or performance metric, like signal-to-interference-and-noise ratio (SINR) of the received signal. In particular, the base station 101 has an array of N antennas 104, where N is an integer greater than or equal to m. The base station prepares one or more transmission signals, represented by the vector xi, for each signal si, where iε{1, 2, . . . , m}. The transmission signal vector xi is determined in accordance with Equation [1]:xi=wi·si  [1]where wi, is the ith precoding, N dimensional transmission weight vector (also referred to as a “transmit precoder”), and each coefficient wj of weight vector wi represents a weight and phase shift on the jth transmit antenna 104. In addition, the term “si” is the data to be transmitted to the ith receiver. Each of the coefficients of weight vector wi may be a complex weight. Unless otherwise indicated, transmission precoding vectors are referred to as “weight vectors,” and reception vectors are referred to as “combining vectors,” though in systems having reciprocal channels (such as TDD systems), a combining vector v at a receiver/subscriber station can be used as both a combining vector (when receiving signals from a transmitter/base station) and a weighting vector (when transmitting to a transmitter/base station).
At the receiver, the received signals detected by the array of antennas 106.i are processed using the appropriate combining vectors 107.i (e.g., v1 and v2). For example, the transmission signal vector x1 is transmitted via a channel represented by a channel matrix Ht, and is received at the receiver 105.1 as a receive signal vector y1=H1Hx1+n1 (where n represents noise and any co-channel interference caused by other subscriber stations). More generally, the received signals for the ith subscriber station 105.i are represented by a ki×1 received signal vector yi in accordance with Equation [2]:
                              y          i                =                                            s              i              *                        ⁢                          H              i              H                        ⁢                          w              i                                +                      (                                                            ∑                                      n                    =                    1                                    m                                ⁢                                                      s                    n                    *                                    ⁢                                      H                    i                    H                                    ⁢                                      w                    n                                                              -                                                s                  i                  *                                ⁢                                  H                  i                  H                                ⁢                                  w                  i                                                      )                                              [        2        ]            where “si” is the data to be transmitted to the subscriber station 105.i, “sn” is the data transmitted to the nth subscriber station 105.n, the * superscript denotes the complex conjugation operator, “HiH” represents the complex conjugate transpose of the channel matrix correlating the base station 101 and ith subscriber station 105.i, wi is the ith transmit weight vector, and wn is the nth transmit weight vector. The superscript “H” is used herein as a hermitian operator to represent a complex conjugate transpose operator. The jth element of the received signal vector yi represents the signal received on the jth antenna of subscriber station 105.i, jε{1, 2, . . . , ki}. The first term on the right hand side of Equation [2] is the desired receive signal while the summation terms less the desired receive signal represent co-channel interference. Finally, to obtain a data signal, zi, which is an estimate of the transmitted data si, the subscriber station 105.i combines the signals received on the k antennas 106.i using a combining vector vi 107.i in accordance with Equation [3]:zi=ŝi=yiHvi,  [3]and then demaps demodulates and decodes the result with processing module 108.i to obtain the data signal, zi.
One difficulty associated with spatial multiplexing is the mapping of one or more codewords onto the physical layers being transmitted by the base station 101, particularly where the number of codewords and available transmit antenna ports at the transmitter can change from one base station to the next. While mapping codewords to layers may be trivial in the case of two antenna ports (since the number of layers equals the number of codewords), the mapping is more complex in the four antenna port case since there are potentially fewer codewords than layers. And even when a mapping structure is adopted, the confines of the mapping structure can create other problems for the operation of the transmitter. For example, a prescribed mapping structure may restrict the ability of the base station to efficiently retransmit signal information to a subscriber station. Accordingly, there is a need for an improved system and methodology for signal processing and control signaling in a MIMO system. There is also a need for a retransmission scheme which may be used within the prescribed mapping structures of a MIMO system. In addition, there is a need for a spatial multiplexing system and methodology which provides a codeword-to-layer mapping approach which overcomes the problems in the art, such as outlined above. Further limitations and disadvantages of conventional processes and technologies will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow.
It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the drawings have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for purposes of promoting and improving clarity and understanding. Further, where considered appropriate, reference numerals have been repeated among the drawings to represent corresponding or analogous elements.